staging: comedi: das16: pass struct comedi_cmd by reference

Change `das16_suggest_transfer_size()` to take a pointer to the `struct
comedi_cmd` instead of passing it by value.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>